This invention relates to a pneumatic detector for nondispersive infrared (NDIR) gas analyzers.
As disclosed in German Pat. No. 1 017 385, a pneumatic detector for a nondispersive infrared gas analyzer includes two gas-filled detector chambers arranged coaxially one behind the other, the detector chambers being provided with radiation-permeable end faces. The two gas-filled chambers are connected to one another via a gas conductive line containing a pneumatic-electrical converter or transducer sensitive to gas pressure or flow.
In such gas analyzers, the two chambers of the pneumatic detector are disposed colinearly with a source of infrared radiation and with a measurement chamber containing the sample gas mixture. The two detector chambers are disposed on one side of the measurement chamber opposite the source of infrared radiation and contain a gas whose presence in the sample gas mixture is to be analyzed and measured. The subject gas in the two detector chambers is present either in pure form or mixed with a nonabsorbing gas.
Upon passage of infrared radiation from the light source through the measurement chamber and the two detector chambers, pressure increases of different magnitudes arise in the two detector chambers. The gas in the first chamber absorbs radiation of a central portion and of flanking portions of an absorption band, while the gas in the second detector chamber absorbs radiation substantially of the band flanks.
The pneumatic-electrical converter or transducer senses the pressure difference between the gas pressures in the first and second detector chambers and generates a measurement signal corresponding to the detected pressure difference.
In such pneumatic detectors the cross sensitivity caused in the subject gas by overlapping absorption bands of other components is greatly reduced or eliminated. Accordingly, the selectivity of the measurement process is generally increased considerably. However, this increase occurs only if the lengths of the two detector chambers are matched so that approximately the same band edge or band flank radiation is absorbed in both. Inasmuch as the amount of radiation of the band flanks absorbed in the detector chambers can vary in accordance with the kind and absorptivity of the participating gases, it is necessary to adjust the absorption conditions when the subject gas is changed. Because such an adjustment is relatively costly, it is desirable to provide simple means for facilitating quantity production of pneumatic detectors of the above-described type.
An object of the present invention is to provide an improved pneumatic detector of the above-described type.
Another, more particular, object of the present invention is to provide such a pneumatic detector in which adaptation of the detector to different measurement conditions, and particularly to different subject gases, is facilitated.
Another particular object of the present invention is to provide such a pneumatic detector which is simple and easy to manufacture in quantity.